Hypertension causes left ventricular hypertrophy (LVH) and increased LV interstitial and perivascular collagen deposition. Increased cardiac collagen may alter function in hypertrophied hearts. Angiotensin converting enzyme inhibition(ACEi) has been shown to reverse LVH in hypertension. This effect has been attributed to blockade of angiotensin II (Ang II) formation and kinin degradation. We have shown that N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP), another natural substrate for ACE, prevents LV monocyte/macrophage infiltration and fibrosis without altering blood pressure (BP) or LVH in hypertension. We believe this is the first demonstration that Ac-SDKP affects cardiac fibrosis; however, the mechanism(s) or receptor(s) by which Ac-SDKP affords cardioprotection are not known, nor whether Ac-SDKP contributes to the cardioprotective effects of ACEi. We hypothesize that (1) Ac-SDKP has an anti-inflammatory and antifibrotic effect on the heart in hypertension via specific receptor(s) linked to a cascade of signal transduction, including MAP kinase, protein kinase C (PKC), NF-kB and TGF-beta, and (2) the effect of ACEi on the heart is mediated in part by Ac-SDKP. In aim new analogues of Ac-SDKP and SDK fragments will be synthesized and tested to identify (1) an ACE-resistant analogue with an inhibitory effect similar to Ac-SDKP that can be radiolabeled and used to characterize Ac-SDKP receptor(s), and (2) an antagonist that specifically blocks the inhibitory effect of Ac-SDKP. In aim II we will determine whether (1) the effect of Ac- SDKP is linked to suppression of TGF-beta, CTGF and Smad expression, and (2) the effect of TGF-beta on collagen type I and/or III expression is affected by Ac-SDKP. We will also examine whether Ac-SDKP inhibits p38 and PKC activation in fibroblasts. In aim IV we will determine whether in vivo Ac-SDKP (1) inhibits inflammatory cell infiltration in the LV, p42/p44, p38 and/or PKC activity, NF-kB,TGF-beta, CTGF and Smads, (2) regresses cardiac fibroblast proliferation, collagen expression and synthesis, (3) increases matrix metalloproteinase activity, and (4) decreases cardiac expression of collagen I and III in rats with long-term hypertension. In aim IV we will examine whether the effect of ACE inhibitionon LV collagen deposition is mediated by Ac-SDKP. This project will provide important new information on the on the cardioprotective effect of ACEi in hypertension and the mechanism of action of Ac-SDKP.